While playing, pause the game and press Left, A, Z, Y.

While playing, pause the game and press Y, A, Right, Down, Down, A, Left, Right.

While playing, pause the game and press Left, A, Z, Y, Down, A, Y, Down.

While playing, pause the game and press Y, A, U, X, C, Left, Y, Up.

While playing, pause the game and press A, Up, Down, Right, Right, Left, X, C.

While playing, pause the game and press Y, A, X, Up, Right, Right, Left, Up.

Here's a complete set of passwords for you.

2: Gun - Energy - Blue Gun - Sandwich - Can Worms
3: Bubblegun - Sandwich - Sandwich - Bubble Gun - Energy
4: 3 Gun - Gun - Missile Gun - 3 Gun - Blue Gun
5: Energy - Bubble Gun - Bullet - Can Worms - Jim
6: Bullet - Sandwich - Gun - Jim - Gun
7: Missile Gun - Blue Gun - Bubble Gun - Bullet - Sandwich
8: Blue Gun - Can Worms - Bullet - Missile Gun - Jim
9: Bullet - Gun - Missile Gun - Bullet - Jim
10: Sandwich - Gun - Jim - Blue Gun - Blue Gun
11: 3 Gun - Bullet - Bubble Gun - Energy - Bubble Gun
12: Missile Gun - Energy - Bullet - Energy - Energy